Not Applicable
Not Applicable
The present invention relates to fluid actuators, in particular to a cushioned fluid actuator, such as a hydraulic cylinder, for use in a multi-stage mast assembly of a lift truck, wherein the actuator has a cushioning assembly which can reduce shock loads encountered in the transition between stages of the multi-stage mast assembly.
Known lift trucks, such as Reach-Fork(copyright), Easi(trademark) Orderpicker, and Pacer(trademark) Truck trucks available from The Raymond Corporation in Greene, N.Y., include a vertically extendible mast supporting a carriage which incorporates xe2x80x9cmast stagingxe2x80x9d to lift a carriage to considerable heights.
xe2x80x9cMast stagingxe2x80x9d refers to a method of lifting/lowering loads on a lift truck carriage in stages (sections). To lift, a fixed hydraulic ram extends until it reaches its end of stroke, whereupon, successive rams (stages) continues the lift. Unfortunately, a shock load is generated as one ram decelerates rapidly at its end of stroke and the next ram accelerates upward. These shock loads can propagate through the lift truck to increase the discomfort of the operator, and can destabilize loads on the carriage.
One known method of mast staging intended to reduce shock loads is disclosed in U.S. Pat. No. 5,022,496. The disclosed method slows the rate of movement of the carriage immediately before and during a stage transition, and then increases the speed of the carriage once the transition is completed. This method changes the speed of the carriage as it moves in a vertical direction which can destabilize a load on the carriage, and cause discomfort to an operator. In addition, this method reduces productivity due to slowing the carriage down through the transition.
Another known method of mast staging intended to reduce shock loads is disclosed in U.S. Pat. No. 5,657,834. The disclosed method incorporates spring elements at the end of each mast stage to cushion the transition between stages. This particular method increases the complexity of the mast assembly, and the difficulty of retrofitting an existing mast assembly. In addition, this method reduces the overall vehicle reliability due to the life of the spring elements.
The present invention provides an actuator for use with a lift truck having an extendible mast. The actuator includes an elongated housing and an axial bore formed in said housing. The bore has a first end and a second end. A ram is slidably and sealingly mounted in the bore for axial movement between an extended position and a retracted position, and has one end disposed in the bore and an opposing end extending out of the bore second end. A pressure ring is slidably and sealingly mounted in the bore between the ram one end and the bore second end, wherein filling the bore with a fluid causes the ram to move from the retracted position toward the extended position, and engagement of the ram one end with the pressure ring traps fluid between the pressure ring and the bore second end to resist movement of the ram toward the extended position.
In another aspect of the invention, the ram one end includes a piston plug body with an axial piston plug bore formed in the piston plug body. The piston plug bore has a first end and a second end, and the piston plug bore first end is in fluid communication with the bore formed in said housing. A piston is slidably and sealingly mounted in the piston plug bore, and has an end extending out of the piston plug bore second end toward the first end of the bore formed in said housing. An accumulator biasing member urges the piston away from the piston plug bore second end.
A general objective of the present invention is to provide an actuator for use with a lift truck multi-stage mast assembly which reduces mast staging shock. This objective is accomplished by providing an actuator with a pressure ring which causes the actuation of a second mast stage prior a first mast stage reaching a fully extended position.
Another objective of the present invention is to provide an actuator for use with a lift truck multi-stage mast assembly which can minimize hydraulic shocks which can occur during a transition between mast stages. This objective is accomplished by providing a ram having on one end a piston plug which can accumulate fluid to minimize hydraulic shocks.
This and still other objects and advantages of the present invention will be apparent from the description which follows. In the detailed description below, preferred embodiments of the invention will be described in reference to the accompanying drawings. These embodiments do not represent the full scope of the invention. Rather the invention may be employed in other embodiments. Reference should therefore be made to the claims herein for interpreting the breadth of the invention.